Till now, there have been many attempts to synthesize nanostructures using an anodic aluminum oxide (hereinafter, referred to as “AAO”) template, including the synthesis of carbon nanotubes in the AAO template by chemical vapor deposition, the formation of sodium nanotubes in the inner wall of the AAO template, and the synthesis of LiMn2O4 nanowires using the AAO template.
In general, one of the advantages of methods for manufacturing (synthesizing) nanostructures using the AAO template is that the manufactured nanostructures have a straight and uniform cylindrical shape and are highly dense. The AAO template does not participate directly in a reaction for producing nanotubes/nanorods, but has many effects on the physical shape of nanostructures.
The nanostructures can be used for various purposes in various industrial fields, and typically serve as energy reservoirs for storing hydrogen.
Hydrogen is recognized as infinite clean resources having little or no possibility of exhaustion, because it can be obtained from water on the earth and is recycled to water after combustion. Because hydrogen (energy) is clean energy that does not generate any pollutant other than water in combustion, it can be used in almost all fields, including various transport means or power plant systems.
However, one problem in the use of such hydrogen energy is that a convenient, economic and safe hydrogen storage system is not yet developed.
One of conventional hydrogen storage methods includes a physical method of compressing and storing hydrogen in a high-pressure container under a pressure higher than 100 atm, but the use of such a high-pressure container on transfer means is very risky in terms of safety. Another physical method for storing hydrogen includes a method of storing hydrogen at a very low temperature lower than the boiling point (20.3 K) thereof. This method is advantageous in that it can store a large amount of hydrogen by reducing the volume of hydrogen stored, but it is very disadvantageous in economic terms, because an additional system (refrigeration system) for maintaining very low temperatures is required.
Meanwhile, a chemical method for storing hydrogen using a hydrogen storage alloy has an advantage of high hydrogen storage efficiency, but it has problems in that, the storage and release of hydrogen are repeatedly carried out, the deformation of the hydrogen storage alloy will be induced by impurities, and thus the hydrogen storage capacity thereof will be reduced with the passage of time. In addition, because the alloy is used as a hydrogen storage medium, the weight per unit volume thereof is increased, thus making it difficult to use the hydrogen storage medium on transport means.
Still other methods for storing hydrogen include methods adsorbing and storing gaseous hydrogen in a solid material. Various reports on the efficiency of methods of storing hydrogen using carbon nanotubes or carbon nanostructure materials among such adsorption methods show hydrogen storage efficiencies much higher than 10 wt %, but such results lack reproducibility, and thus many studies are still ongoing.
Accordingly, many studies are currently ongoing to develop a hydrogen storage method, which ensures a hydrogen storage efficiency of more than 6.5%, a hydrogen storage target set by the US Energy Department of Energy (DOE), and safety and economic efficiency, and eliminates the above-mentioned problems.
Lithium secondary batteries, which are the energy sources of electronic information telecommunication devices and small-size devices, comprise positive and negative electrodes made of a material capable of reversibly intercalating and deintercalating lithium ions, are prepared by pouring an organic electrolyte or polymer electrolyte between the positive electrode and negative electrode, and produce electrical energy by oxidation/reduction reactions when lithium ions are intercalated into and deintercalated from the positive and negative electrodes. As the negative electrode active material of the lithium secondary battery, a carbon-based material such as amorphous carbon or crystalline carbon is mainly used. The positive electrode active materials include LiCoO2, LiNiO2 and LiMn2O4, which have a layered structure or include a tunnel-shaped space in the crystal thereof so as to permit the absorption and release of lithium ions. Manganese oxide-based based positive electrode active materials are unstable to temperature variations, but they are frequently used, because they are inexpensive, environmentally harmless, and show high energy density, compared to other active materials.
In lithium ion secondary batteries, the charge/discharge capacity of the positive electrode varies on the size and structure of particles. Specifically, as the particle size of the active material is decreased, the diffusion rate of lithium ions can be increased to increase the charge/discharge capacity of the positive electrode. Also, even when the active material has a particle structure in which the diffusion of lithium ions easily occurs, the charge/discharge capacity of the positive electrode itself can be increased. Moreover, the stability of the crystal structure has a close connection with reversibility, and has a close connection with the cycle life of the battery. Accordingly, preparation of powder, which contains no foreign matter and has excellent crystallinity, is a key technology for determining battery performance.
However, prior methods of preparing composite metal oxides have problems in that various complex steps are carried out and many facilities and long time are required. Also, in the prior methods of synthesizing composite metal oxides, the temperature of the synthesis is high, the particle size of reaction materials is relatively large, it is difficult to control the physical properties (e.g., particle shape or surface characteristics) of active materials, and limited starting materials such as oxides must be used. Accordingly, if pure LiMn2O4.based compounds in the form of nanotubes can be obtained by a simple preparation method, they can be applied as the positive-electrode active material of lithium secondary batteries.
Till now, there has been no example in which compounds in the form of nanotubes were used as the positive-electrode active material of secondary batteries.